1. Field of the Invention
The present invention generally relates to superconducting filter devices and, more particularly, to a superconducting filter device used for a receiver amplifier provided in a base station of a portable telephone communication system.
2. Description of the Related Art
In recent years, with the explosive development and popularization of portable telephones, there is a demand for development of a signal transmission technique that enables a high-speed, large-capacity signal transmission. As a technique which satisfies such as demand, there is suggested a technique using a superconducting filter as a frequency band filter used for a receiver amplifier provided in a base station of a portable telephone communication system.
A superconducting material usable as the frequency band filter is suitable for a microstrip line type filter since a surface resistance thereof is much smaller than that of a normal electrically conductive material even in a high-frequency range. Problems lie in putting such a superconducting filter in practical use, such as, for example, a problem in producing a low-temperature environment, have been greatly eliminated.
Recently, a superconducting filter for a receiver, which uses a superconductor, has been put in practical use. By using such a superconducting filter also for a transmitter circuit, it can be expected to eliminate distortion generated in an amplifier.
For example, Japanese Laid-Open Patent Application No. 2001-102809 suggests a method of adjusting a frequency band by using a separation plate that is located between and above adjacent resonators so as to adjust coupling between the resonators by shifting the separate plate upward or downward.
Additionally, “High-Tc Superconducting High-Power Filters Using Elliptic-Disc Resonators”, 1998 electronics information communication electronics society, electronics society meeting papers, p.p. 391–392 discloses an elliptic resonator used for adjusting frequency band of a superconducting filter. Further, “Elliptic-Disc Filters of High-Tc superconducting Films for Power-Handling Capability Over 100 W”, IEEE Trans. Microwave Theory Tech., vol. 48, No. 7, pp. 1256–1264, July 2000 also discloses an elliptic resonator used for adjusting frequency band of a superconducting filter.